The present invention relates to a forward converter, and more particularly to a third winding reset circuit on a transformer core.
The forward converter is the most idealist topology in the power conversion field, it can be widely used in several applications such as AC-to-DC, DC-to-DC and other power electronic equipments. Conventional forward converters have some problems such as the voltage stress, the work duty which can""t be over 50% under some conditions. Therefore, many new reset circuit technologies have been developed. The third winding reset circuit proposed before is not approved entirely by the public. Applicant believes that it could be improved and the following problems can be solved. FIGS. 1a and 1b, show a convention forward converter and timing diagram, FIGS. 2a and 2b, are the conventional third winding reset for a forward converter and timing diagram.
As shown in FIG. 1a a conventional forward converter, power switch Sm is coupled in series with the primary winding P1 of a transformer. Each time, the power switch Sm is turned on and off is controlled by the gate driving signals of the pulse-width-modulated (PWM) controller. The secondary side of the converter has a forward rectifier D1 coupled to the secondary winding S1 of the transformer, a free-wheeling rectifier D2 and an output filter consisting of an output choke Lout and an output capacitor Cout. The output filter transfers DC energy to RL load from the primary side DC source V1.
When power switch Sm is turned on, the input voltage V1 is applied across the primary winding P1 of the transformer T1, and the voltage is coupled to secondary winding S1. The positive end of the secondary winding S1 is turned positive, and the forward rectifier D1 is turned on, the free-wheeling rectifier D2 is turned off, the forward power current flows to output choke Lout, output capacitor Cout and load RL.
When power switch Sm is turned off, the positive end of the secondary winding S1 is turned negative. The forward rectifier D1 is turned off and the free-wheeling rectifier D2 must be turned on. Because the power current of output choke Lout must be forwarded continually to the output load RL by the free-wheeling rectifier D2.
FIG. 1b shows the timing diagram of the circuit of FIG. 1a. The conventional forward converter has some problems, such as, the voltage stress and the work duty. If the work duty design is over 50%, the voltage stress will be dangerous. From t=t0 to t2, the power switch Sm is turned off, during t0 to t1 time diagram, across Drain-Source pin of the power switch is twice more than V1 voltage stress., so that Drain-Source pin of the power switch must endure twice more than V1 voltage. In this case, the material cost will be high. If the VT product of transformer between t0 and t1 can be kept constantly during the time period of t0 to t2, the voltage stress will be reduced to a minimum value.
As shown in FIG. 2a a conventional third winding reset of the forward converter, the power switch Sm is coupled in series with the primary winding P1 of a transformer. Each time, the power switch Sm is turned on and off is controlled by the gate driving signals of the pulse-width-modulated (PWM) controller. The DC source V1 of the primary side is coupled in parallel with the DL network, and comprises a diode D3 and a third winding S2. The DL network is used to reset the magnetizing current of the primary winding of transformer T1 and the current will be recycled to DC source. The secondary side of the converter has a forward rectifier D1 coupled to the secondary winding S1 of the transformer, a free-wheeling rectifier D2 and an output filter consisting of an output choke Lout and an output capacitor Cout. The output filter transfers DC energy to RL load from the primary side DC source.
When the power switch Sm is turned on, the input voltage V1, is applied across the primary winding P1 of the transformer T1, and the voltage is coupled to secondary winding S1. The positive end of the secondary winding S1 is turned positive, now, the forward rectifier D1 is turned on, the free-wheeling rectifier D2 is turned off, the forward power current flows to output choke Lout, output capacitor Cout and load, RL. The positive end of the third winding S2 is turned positive voltage, the diode D3 is turned off.
When power switch Sm is turned off, the positive end of the secondary winding S1 is turned into negative. The forward rectifier D1 is turned off and the free-wheeling rectifier D2 must be turned on. Because the current of output choke Lout must be forwarded continually to the output load RL by the free-wheeling rectifier D2. The negative end of the third winding S2 is positive voltage, the diode D3 will be turned on, the magnetic flux from the transformer will be reset, the magnetizing current flows back to DC source by D3 and the clamp voltage will be twice to V1 in the power switch Drain-Source.
FIG. 2b shows the timing diagram of the circuit of FIG. 2a. The conventional third winding of forward converter still has some problems such as, the Vds of power switch voltage is still high, and the work duty designs only 50%. If the design of work duty is over 50%, the voltage will be clamped, according to voltage-second balance, the transformer will be saturated when the drain current Id of power switch has moved to high peak and destroys power switch Sm.
From t=t0 to t2, the power switch Sm is turned off; during t0 to t1 time diagram, Drain-Source of the power switch is across twice to V1 voltage stress. So that, the component must also meet twice to V1 for power switch Vds. In this case, the material cost is still high.
The aforementioned issue has two problems, voltage stress and work duty, which will be improved effectively by this third winding reset circuit present invention. The present invention consists of a rectifier diode Dr, a storage capacitor Cs, an auxiliary switch Sa, a storage inductor La and a free-wheeling diode Df. When power switch Sm is turned off, the rectifier diode Dr provides a forward conduction path to transfer magnetizing energy from a transformer and this energy flows to storage capacitor Cs. In the same time, the auxiliary switch Sa is turned on by the negative end of the third winding S2, must be high level voltage, to provide a path to discharge the energy through the capacitor Cs and the storage inductor La to the DC source. The free-wheeling diode Df provides a path to discharge the current of storage inductor La when the power switch Sm is turned on and the auxiliary switch is turned off.
The third winding provides a new reset circuit technique to reset magnetized energy easier from the transformer, effectively to reduce the voltage stress, and to surpass work duty 50% maximum limited of the conventional third winding forward converter. So the new third winding reset circuit structure is simple and manufactured at low cost.